Allow disabling "select existing" on relationship/upload fields (create/upload-only)

[pauksztello]

It would be useful to be able to disable the "choose existing" picker on relationship and upload fields, so editors can only create/upload a new document rather than pick an existing one.

Today these fields always offer both options — select an existing related document or create/upload a new one. There's an admin.allowCreate option to remove the create half, but no equivalent to remove the select-existing half.

Use cases

Media that must always be unique. A common one: an upload field pointing at a Media collection where each upload should be its own file. Right now the editor can browse and reuse any existing item in the media library, but in some setups reusing media is undesirable — every entry should force a fresh upload (e.g. per-document assets that shouldn't be shared, or to avoid editors accidentally attaching the wrong existing file). There's currently no way to hide the existing-media browser and only allow uploading.

Prior art

• The Join field gained admin.allowCreate: false in feat: prevent create new for joins #8929 (Ability to disable Create/Add new on Join Field #8892), establishing create/select